In recent years, betaines of formula I have gained increasing importance for the preparation of cleansing agents for the body. they combine outstanding cleansing properties with good skin compmatibility. In aqueous solution, the betaines form a stable, dense foam, which does not collapse even in the presence of soap.
The synthesis of these betaines is described in many patents, of which U.S. Pat. No. 3,225,074 is cited as representative. In general, the appropriate tertiary fatty acid amidamine of the general formula II is reacted with the alkali salt of an .omega.-halogencarboxylic acid, usually the sodium salt of chloroaceic acid. The reaction takes place in the aqueous medium. The alkali chloride, formed during the reaction, generally is not removed from the solution.
The betaines of the state of the art are most marketed in the form of their 30% by weight aqueous solutions. There has been no lack of attempts to produce more concentrated betaine solutions in order to lessen the costs of transport and storage. However, if water is withdrawn from the betaine solutions obtained according to the state of the art, the viscosity of the solutions increases rapidly. Solutions of betaines based on hydrogenated coconut fatty acid become pasty when they contain about 35 to 37% by weight of betaine and solidify on further dehydration. The concentration, above which the betaine solutions no longer are flowable, is affected by the number of carbon atoms of the fatty acid R.sup.1 COOH. The longer the chain length of the fatty acid or of the fatty acids of the mixture used to prepare the betaine, the more rapidly does the viscosity of the betaine solutions increase as the concentration increases. Unsaturated fatty acids with the same number of carbon atoms result in betaine solutions of lower viscosity.
Those skilled in the art know that the viscosity of an aqueous surfactant solution also increases as the concentration is increased. Frequently, however, it turns out that, when a concentration of about 60 to 70% by weight is exceeded, the viscosity at first falls off to a minimum as the concentration is increased further, only to then increase once again strongly. To explain this viscosity anomaly, it is assumed that a G phase with lamellar structure is formed in the solution (Soap, Perfumery, Cosmetics, 1982, pages 507 to 509). However, such a behavior could not be observed with betaines. On further removal of water, the solidified products do not liquefy once again.
In the German Offenlegungsschrift No. P 36 13 944.0-42, a method is described for the preparation of a flowable and pumpable solution containing at least 70% by weight of betaine of formula I, in which
(a) the ammonium salt is used as salt of the halogenated carboxylic acid,
(b) the quaternization is carried out in a polar organic solvent, which must not contain more than 20% by weight of water,
(c) after the quaternization, any water contained is distilledc off azeotropically and the precipitated ammonium halide is removed, after which
(d) the solvent is distilled off partly or completely and
(e) before, simultaneously with or after the distillation, the concentration of the betaine in the solvent or solvent mixture desired for the particular application is adjusted to the desired value.
For this method then, the quaternization must be carried out in a polar organic solvent. The concentrated solution of the betaine, as product of the process, is present in a solution in a solvent or solvent mixture, which is required for the particular application.
However, there continues to be a need for producing aqueous solutions of betaines of formula I in the highest possible concentrations which in spite of their comparatively high concentration are flowwable and pumpable and, preferably, free of solvents other than water. A low viscosity of the aqueous solutions is required especially so as to be able to convey and meter the aqueous betaine solutions during further processing. There is, moreover, an appreciable economic interest in lowering the packaging, transport and storage costs and in reducing the handling expenses. It is of particular importance that the concentrated aqueous solutions can be dispersed in water duirng dilution without forming a gel.